Detecting errors in optical messages

ABSTRACT

In example implementations, an apparatus includes a bus waveguide, a plurality of optical gates coupled to the bus waveguide and an injection coupler. The bus waveguide receives a plurality of constraint signals. Each optical gate outputs an internal state via a local phase shift when at least one of the plurality of constraint signals has a wavelength that matches a respective resonant wavelength. The injection coupler combines the at least one of the plurality of constraint signals with additional constraint signals that are injected. An error is detected in a bit of a message when an overall phase shift has occurred to the at least one of the plurality of constraint signals causing a power level to exceed a power level threshold of an optical gate when the at least one of the plurality of constraint signals constructively interferes with the additional constraint signals that are injected.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Contract No.N66001-12-2-4007, awarded by Defense Advanced Research Projects Agency.The government has certain rights in this invention.

BACKGROUND

Optical communication is the leading solution for high speed, highbandwidth exchange of digital information. Optical communicationstransmit information over long distances using light signals over lighttransmitting mediums, such as optical fibers and waveguides.

Information can be carried by the light signals using different types oflight sources as optical transmitters (e.g., light emitting diodes(LEDs), infrared light, lasers, and the like). A light signal can beencoded using different types of modulation schemes to vary the opticalphase or intensity of the light signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example apparatus of the presentdisclosure;

FIG. 2 is a diagram of an example operation of the apparatus;

FIG. 3 is a flow diagram of an example method for detecting an error inan optical data package; and

FIG. 4 is a flow diagram of another example method for detecting anerror in an optical data package.

DETAILED DESCRIPTION

The present disclosure discloses systems and techniques for providing anall optical error detecting and correcting solution for informationencoded in an optical data package. The error detection and correctioncan occur even with a plurality of different wavelengths beingtransmitted within the same system.

As discussed above, optical communication systems transmit informationover long distances using light signals over light transmitting mediums,such as waveguides. Information can be carried by the light signalsusing different types of light sources as optical transmitters. Theinformation can be encoded using different types of modulation schemesto vary the intensity or optical phase of the light signal.

Optical phase may be defined relative to that of a stable referencesignal of the same frequency and can take on values between 0 and 2π.Phase values outside that interval can be identified with their valuemodulo 2π.

Two optical signals of the same wavelength and comparable powerinterfere constructively when they have a similar optical phase. Twooptical signals of the same wavelength and comparable power interferedestructively when their relative optical phase is approximately π.

Current solutions for optical and electronic communication transmit awhole data package, i.e., a fixed length sequence of many bits, at once.However, errors can occur within the package. Current solutions use ahigh optical power or a lower modulation rate in order to achieve a highsignal to noise ratio (SNR), or equivalently a low bit-error rate (BER).Current solutions use the entire data package to be fully re-transmittedwhen an error is detected, which incurs a high latency penalty.

In addition, the error processing is carried out electronically, whichmay use latency and energy-intensive conversion from optical signals toelectrical signals and back. As a consequence, current opticalcommunications technology implements electrical error correctiontechnologies that use a high amount of energy.

The present disclosure, provides an all optical solution to detectingand correcting errors in optical data packages transmitted over lightsignals. The techniques described by the present disclosure may useexisting optical communication schemes such as wavelength divisionmultiplexing (WDM). As a result additional wavelength conversions, whichcan be expensive, are eliminated. The WDM scheme also enables sharingwaveguides and gates in the circuit between different optical modes.

In one example, the examples of the present disclosure leverage thecoding schemes associated with linear block codes. In one example, themethods associated with low density parity check (LDPC) codes may beleveraged by the all optical solution to provide autonomous errordetection and correction. An example for a linear block code is given bya Hamming (7,4) code. A review of the Hamming (7,4) methods may providethe reader some background in the coding theories implemented by the alloptical solutions described herein. In addition, a review of LDPC's, andspecifically their subclass named “Expander Codes,” may provide thereader some background with the iterative message passing based decodingscheme that is implemented by the apparatus and systems describedherein.

A linear block code is fully described by a binary parity check matrixH. A data package “x” can be checked for errors by computing the errorsyndrome c=H×(modulo 2). When any bit in c is non-zero, an error hasoccurred.

FIG. 1 illustrates an example apparatus 100 of the present disclosure.The apparatus 100 may include a bus waveguide 102, a plurality ofoptical gates 104-1 to 104-n (herein also referred to individually as anoptical gate 104 or collectively as optical gates 104), and an injectioncoupler 106. The bus waveguide 102 may either wrap around into a closedloop or have a reflective end to form a large optical resonator aroundthe optical gates 104.

In one implementation, the optical gates 104 may be deployed, orarranged, to implement a decoding scheme associated with a linear blockcode. In one example, the linear block code may be an LDPC codingscheme.

Each optical gate 104 stores one bit of the optical data package. In oneexample, the optical data package may include the message represented incoding theory terms. The initial state of the optical gate 104 may beinitialized in various ways depending on the application and how theoptical gate 104 is actually implemented.

The optical gates 104 may interact with a set of constraint signals 108of different wavelengths that propagate in the bus waveguide 102. Theconstraint signals 108 may be optical signals that are coupled into thebus waveguide 102 via the injection coupler 106 to provide an alloptical solution to autonomously detect and correct bit errors in amessage. In other words, the bit errors can be detected and corrected inthe optical data package without converting the optical data package toan electrical signal and back to the optical data package.

In one example, there may be one constraint signal 108 with a separateoptical wavelength per parity check for the LDPC. Each constraint signal108 may be initially injected with a comparable input power.

In one implementation, the apparatus 100 may determine an error hasoccurred in a bit whenever at least one constraint signal 108 picks up aparticular total phase shift after travelling through the optical gates104 and the bus waveguide 102.

For example, if a constraint signal 108 of a fixed wavelength enters thebus waveguide 102 at the injection coupler 106 with a phase of 0 andreturns to the injection coupler 106 with a total roundtrip phase of 0,then it may be determined that a bit error has occurred in the message.

For each constraint signal 108, an optical roundtrip phase of 0indicates that at least one of the data bits participating in thatconstraint signal 108 has an error. For each constraint signal 108, theconstraint bit may be defined to be 1 when the roundtrip phase is 0 andthe constraint bit may be defined to be 0 if the roundtrip phase is π.

The full set of constraint bits gives the error syndrome. When allconstraint bits equal 0, the input data encoded in the optical gates 104is in a valid codeword.

For each possible error syndrome that contains non-zero bits, there is amost likely valid codeword. The system may be designed such that biterrors are corrected and the bit sequence of the optical gates 104converges to the most likely valid codeword.

When a constraint signal 108 has a roundtrip phase of 0, the constraintsignal 108 may constructively interfere with another injected constraintsignal 108 at the injection coupler 106. This may lead to a buildup ofoptical power at that particular wavelength traveling around the buswaveguide 102.

The locations of bits that have a high probability of an error may bedetermined autonomously by the optical gates 104. Those optical gates104 that receive power from their resonant constraint modes in excess ofa respective power level threshold are likely to contain an error. Forexample, each separate wavelength may correspond to one constraint c_(j)for the code word of the LDPC coding scheme, as discussed below withreference to FIG. 2. When a bit error occurs at an optical gate 104,each constraint signal 108 associated with the optical gate 104 may bein an error state causing the power level threshold of the optical gate104 to be exceeded. This may cause the optical gate 104 to switch therespective internal state, which changes the roundtrip phase of itsresonant constraint signals 108, which leads to destructive interferenceat the injection coupler 106, which reduces the total power level of theoptical gate 104.

The message may be automatically corrected as the apparatus 100continues to operate until a total power level of the apparatus 100reaches a steady state or equilibrium. In other words, the bit valuestored at each one of the optical gates 104 when the total power levelof the apparatus 100 reaches the steady state may be the corrected bitvalues for a message that is encoded in the initial state of the opticalgates 104.

FIG. 2 illustrates an example of the apparatus 100 that implements acoding scheme that uses six constraints 202-1 to 202-6 (herein referredto individually as a constraint 202 or collectively as constraints 202)also labeled as boxes c1 to c6. In one implementation, the plurality ofoptical gates 104 may be illustrated as nodes Z1 to Z8. FIG. 2illustrates an example implementation using eight optical gates 104-1 to104-8 (herein referred to individually as an optical gate 104 orcollectively as an optical gates 104).

In one implementation, the optical gates 104 may be deployed to operateon the constraint signals 108 as desired. For example, the optical gates104 may have at least one physical input coupler and at least onephysical output coupler through which the constraint signals 108 canenter and exit the optical gates 104. The input and output coupler canbe realized through the same physical element.

The optical gates 104 may interact with a particular set of resonantoptical wavelengths. For example, each one of the constraints 202-1 to202-6 may have a wavelength that is associated with a subset of theoptical gates 104-1 to 104-8. For example, the wavelength associatedwith the constraint 202-1 may be a resonant wavelength for the opticalgates 104-1, 104-3, 104-4, 104-6 and 104-7. The wavelength associatedwith the constraint 202-2 may be a resonant wavelength for the opticalgates 104-2, 104-3, 104-5 and 104-7. The association between theremaining constraints 202-3 to 202-6 and the optical gates 104-1 to104-8 is illustrated in FIG. 2 by the lines or edges that are drawnbetween them.

The optical gates 104 may not couple to a constraint signal 108 that isnot at one of the resonant wavelengths. In other words, the constraintsignals 108 that are off-resonant may simply pass through the opticalgates 104 unaltered or with a constant phase shift α (ideally α=0).

When the total incident optical power is below a power threshold levelof the optical gates 104, the optical gates 104 have at least two stableinternal states x∈{0,1}. As a result, the optical gates 104 can carry abit of information. The internal state x∈{0,1} can be deterministicallyset through particular combinations of input signals. The internal statex∈{0,1} can be read out continuously or when the optical gates 104inputs are in a particular readout-configuration.

When the total incident optical power in resonant constraint modes isbelow the power threshold level, the optical gates 104 operate on theconstraint signals 108 that are resonant by modifying the optical phaseφ of the constraint signals 108 conditional on its internal state x asφ→xπ+βB. In other words, when the stored bit is x=0, the optical phaseis unaltered except for a state independent phase shift β (ideally β=0),and when the stored bit is x=1, the optical phase receives an additionalπ phase shift relative to the x=0 case.

When the total incident optical power is at or above the power thresholdlevel, the internal bit state x can become undefined until the totalpower is lowered again, at which point it randomly becomes either 0or 1. Alternatively, the internal bit state x can switchdeterministically to the logical inverse of the current statex_(k)→¬x_(k), where k=1 to N, for N bits.

The optical gates 104 may be realized by exploiting any dispersivenon-linearity that works across different wavelengths such as theoptical Kerr-effect, Free-carrier dispersion, or the thereto-opticeffect. The selective wavelength sensitivity used to interact with asubset of the constraint signals 108 can be achieved by employingresonant structures. With these basic ingredients, engineering abi-stable optical system that shifts a subset of passing wavelengths bya conditional phase shift of π or 0 may be implemented.

The remaining problem may be how to ensure that the optical gate 104switches its internal state independently of the previous state above acertain total incident power. There are several ways to achieve this,but one example may be to add additional non-linear elementsimplementing all-optical logic. Another approach, with potentiallysmaller hardware overhead may be to start from one such bi-stabledevice, duplicate it, and connect both together in such a way that thetwo bi-stable devices will always assume the opposite internal state. Ifimplemented correctly, the symmetry of this construction may realize thedesired bifurcation behavior at the threshold power.

Given these properties of the optical gates 104, the constraint signalor signals 108 may propagate through the bus waveguide 102. Whenever theconstraint signal 108 passes through an optical gate 104 that has anassociated resonant wavelength that is equal to the wavelength of theconstraint signal 108, the constraint signal 108 picks up a statedependent phase shift E_(j)→E_(j) e^(i(x) ^(k) ^(π+β))=E_(j) (−1)^(x)^(k) e^(iβ), where e is Euler's number, j=1 to N-M where M is a numberof actual bits of a message encoded into N bits and the values x_(k) andβ are defined above.

It should be noted that a single phase shift π corresponds tomultiplication by a factor of (−1). The resonance wavelengths of theoptical gates 104 may be designed such that an optical gate 104 isresonant with a wavelength of the constraint signal 108 if H_(jk)=1,where H_(jk) is a binary valued parity check matrix of the LDPC. Thematrix H_(jk) may specify which bits participate in which constraints asrepresented by the example associations drawn in FIG. 2 between theconstraints 202-1 to 202-6 and the optical gates 104-1 to 104-8. Inother words, the matrix H_(jk) may be used to pre-define the resonantwavelengths associated with each one of the optical gates 104.

After a full roundtrip inside of the bus waveguide 102, the constraintsignal 108 may arrive at the injection coupler 106 as E_(j)→ηE_(j)(−1)^(c) ^(j) , where the exponent of (−1) equals the constraintchecksum c_(j)=Σ_(k=1) ^(N)H_(jk)x_(k) and 0<η≤1 is a constantattenuation factor (ideally η=1).

Before arriving at the injection coupler 106, the constraint signal 108may go through a final phase shifter 204 to compensate all stateindependent phase shifts from the optical gates 104. In addition, thefinal phase shifter 204 may be used to enforce that constraint modescorresponding to an error build up power through constructiveinterference. In one example, the final phase shifter 204 may apply aphase shift of n to all constraint signals 108 that pass through the buswaveguide 102. The final phase shifter 204 may ensure that theconstraint signal 108 destructively interferes with input constraintsignals 108 at the injection coupler 106 when the overall phase shift is0 indicating no error. Alternatively, the final phase shifter 204 mayensure that the constraint signal 108 constructively interferes with theinput constraint signals 108 at the injection coupler 106 when theoverall phase shift is π indicating an error has occurred.

At the injection point, the returned constraint signal 108 may interferewith the amplitudes of additional constraint signals 108 being injectedat the injection point such that in equilibrium (e.g., achieved after afew round trips) the amplitudes of the constraint signals 108 inside thebus waveguide 102 satisfy E_(j)=√{square root over (1−t²)}U_(j)−t ηE_(j)(−1)^(c) ^(j) , t is the amplitude transmissivity and is 0<t<1. Theamplitude transmissivity t may characterize the transmission through theinjection coupler 106 and the external signal injection amplitude U_(j).

The injection coupler 106 may be designed to have the amplitudetransmissivity to be very close to 1. This may imply that solving forE_(j) may lead to different signal mode amplitudes E_(j) conditional onthe constraint value c_(j) given by the Equations below:

$E_{j} = {{\frac{\sqrt{1 - t^{2}}}{1 + {t\;{\eta\left( {- 1} \right)}^{c_{j}}}}U_{j}} \approx \left\{ \begin{matrix}{{\frac{1}{1 + {t\;\eta}}U_{j}\mspace{14mu}{for}\mspace{14mu} c_{j}} \equiv {0\mspace{14mu}\left( {{mod}\mspace{14mu} 2} \right)}} \\{{\frac{1}{1 - {t\;\eta}}U_{j}\mspace{14mu}{for}\mspace{14mu} c_{j}} \equiv {1\mspace{14mu}\left( {{mod}\mspace{14mu} 2} \right)}}\end{matrix} \right.}$for high transmissivity and small roundtrip losses 0≤1−tη<<1. Thus, thesignal mode for a violated constraint c_(j)=1 builds up far more powerinside the bus waveguide 102 than for a satisfied constraint c_(j)=0 bya factor of

$\frac{E_{j}^{2}\left( {c_{j} = 1} \right)}{E_{j}^{2}\left( {c_{j} = 0} \right)} = {\left( \frac{1 + {t\;\eta}}{1 - {t\;\eta}} \right)^{2} ⪢ 1.}$This may happen simultaneously for all constraint signals 108.

For a given optical gate 104 whose bit state has been corrupted due to atransmission error, with high likelihood most of the constraints (e.g.,parity checksums) that that the optical gate 104 participates in, willindicate an error c_(j)=1 The optical gates 104 may be resonant with adifferent subset of resonant wavelengths of the constraint signals 108.Thus, the optical gates 104 with corrupted bits will, with a highlikelihood, be resonant with the largest number of constraint signals108 in the error state. Therefore, the optical gates 104 that containcorrupted bits may receive coherent feedback in the form of higheroptical input power, driving them to reach the respective powerthreshold level and switch the respective internal state. When thishappens for a given optical gate 104, all constraint modes resonant withthat optical gate 104 will flip the respective optical phase and, thus,change the respective power level.

Constraint checksums may be computed by the constraint signals 108 thatpick up a total phase shift around the bus waveguide 102. Message bitsstored in an optical gate 104 may be induced to switch when many of theconstraints associated with one of the optical gates 104 cause theoptical gate 104 to build up power due to being in the error state, asdescribed above.

In equilibrium, the transmitted external injection amplitudes U_(j)′ maybe given by the equation below:

$U_{j}^{\prime} = {\left( \frac{t + {\eta\left( {- 1} \right)}^{c_{j}}}{1 + {t\;{\eta\left( {- 1} \right)}^{c_{j}}}} \right)U_{j}}$If the transmissivity is matched to be equal to the internal roundtripattenuation (e.g., t=η), then the transmitted injection amplitude mayvanish, while the constraint is violated. By monitoring the totaltransmitted power level P′∝Σ_(j=1) ^(N−M)|U′U_(j)′|², it can bedetermined when the apparatus 100 has converged. The stored bit value ofeach gate 104 when the apparatus 100 has converged represent the correctbit value for each bit of the data package.

FIG. 3 illustrates a flow diagram of an example method 300 for detectingan error in an optical data package. In one example, the blocks of themethod 300 may be performed by the apparatus 100.

At block 302, the method 300 begins. At block 304, the method 300receives a constraint signal at an injection point into a series of aplurality of optical gates. In one example, the constraint signal maycomprise a plurality of constraint signals having an equivalent inputpower level, but different wavelengths.

In one example, the optical gates may be arranged to implement a linearblock code coding scheme (e.g., an LDPC coding scheme). The number ofoptical gates that are deployed may be equal to a total number of bitsin the data package (e.g., actual message bits plus redundant bits usedto implement the coding scheme).

As described above, each one of the optical gates may store a binary bitvalue of 0 or 1. The bit value that is stored may be read out by theconstraint signal as a phase shift. For example, a value of 0 may beread out when no phase shift is applied and a value of 1 may be read outwhen a phase shift is applied.

A phase shift may be applied when a wavelength of the constraint signalmatches a resonant wavelength of an optical gate. For example, eachoptical gate may be associated with a plurality of resonant wavelengths.Said another way, a plurality of different subsets of optical gates maybe selected for each constraint of a respective check sum. The pluralityof resonant wavelengths associated with each optical gate may bepredetermined based on binary parity check sum constraints that areassociated with each one of the optical gates (e.g., the binary valuesof the matrix H described above).

At block 306, the method 300 determines that the constraint signalincurred an overall phase shift through the series of the plurality ofoptical gates, wherein the overall phase shift is indicative of acorrupted bit of a message. For example, for a particular constraintsignal, the overall phase shift should be zero when travelling throughthe series of optical gates. For example, the optical signal may beginwith a phase of 0 or 2π. A first optical gate may apply a phase shift ofπ, a second optical gate may apply a phase shift of π and a thirdoptical gate may apply no phase shift. Thus, the constraint signal hasno error since the overall phase shift is 0. However, to ensure that theconstraint signal destructively interferes with input constraint signalsat the injection coupler, a final phase shifter may apply a global phaseshift of π. Thus, the constraint signal begins with a phase of 0 andreturns to the injection coupler with a phase of π. Thus, noconstructive interference occurs and the power level of the constraintsignal is decreased in the next roundtrip.

However, if the constraint signal incurs an overall phase shift of πfrom the optical gates and returns to the injection coupler with a phaseof 0 after the final phase shifter is applied, then an error hasoccurred. The method 400 discussed below may determine that a bit of themessage is corrupted.

At block 308, the method 300 increases a power level in response to thedetermining that the constraint signal incurred the overall phase shift.For example, when the constraint signal arrives at the injection couplerwith a phase shift of 0 (e.g., an overall phase shift of π plus a finalphase shift of π that is applied to all constraint signals), theconstraint signal may interfere constructively at an injection couplerwith input constraint signals that are being injected into the buswaveguide. Thus, the returning phase shifted constraint signal wheninterfered with the other constraint signals may cause the overall ortotal power of the system to increase.

At block 310, the method 300 repeats the determining and the increasinguntil the power level exceeds a power level threshold of at least one ofthe plurality of optical gates indicating that the at least one of theplurality of optical gates is a source of the corrupted bit. Forexample, when the block 306 and the block 308 are repeated, the totalpower of the system may increase until it exceeds the power levelthreshold of one of the optical gates. The optical gate that has a totalpower level that exceeds the respective power level threshold may beidentified as the bit that has the error or is storing the corruptedbit.

At block 312, the method 300 switches an internal state of the at leastone of the plurality of optical gates in response to the power levelthreshold being exceeded. For example, if the value of the optical gatethat is causing the bit error is 1, the value may be switched to 0 toreduce the total power level back below the power level threshold.

In one example, the value may be switched deterministically or may beswitched randomly. In one example, the determining, the increasing, therepeating and the switching in blocks 306, 308, 310 and 312,respectively, may be repeated until the total power level reaches asteady state. The values stored in each one of the optical gates whenthe total power level reaches the steady state may be the valueassociated with each bit of the message. At block 314, the method 300ends.

FIG. 4 illustrates a flow diagram of an example method 400 for detectingan error in an optical data package. In one example, the blocks of themethod 400 may be performed by one of the optical gates 104.

At block 402, the method 400 begins. At block 404, the method 400receives, at an optical gate, a constraint signal. In one example, theconstraint signal may be one of a plurality of constraint signals thatis received. The plurality of constraint signals may have an equivalentpower level, but different wavelengths.

In one example, the optical gate may be arranged to implement a linearblock code coding scheme (e.g., an LDPC coding scheme). The number ofoptical gates that are deployed may be equal to a number of bits (e.g.,actual message bits plus redundant bits used to implement the codingscheme).

At block 406, the method 400 determines that a wavelength of theconstraint signal matches a resonant wavelength of a resonator of theoptical gate. In one example, the resonant wavelength of the resonatormay be selected based on participation of the bit value stored in anoptical gate with an associated constraint of a respective check sum.

At block 408, the method 400 may apply a phase shift stored in theresonator to the constraint signal that contributes to an overall phaseshift of the constraint signal. The phase shift imparted by the opticalgate on a constraint signal that is resonant may be representative of abinary bit value of 0 or 1 that is stored in the optical gate. The bitvalue that is stored may be read out by the constraint signal as a phaseshift. For example, a value of 0 may be read out when no phase shift isapplied and a value of 1 may be read out when a phase shift is applied.

An overall phase shift from all optical gates is indicative of acorrupted bit of a message. For example, for a particular constraintsignal, the overall phase shift should be zero after travelling throughthe series of optical gates, or equivalently it after passing throughthe final phase shifter. For example, the optical signal may begin witha phase of 0 or 2π. A first optical gate may apply a phase shift of it,a second optical gate may apply a phase shift of it and a third opticalgate may apply no phase shift. Thus, the optical signal begins with aphase of 0 and passes through the optical gates with an overall phaseshift of 0, but then passes through the final phase shifter and arrivesback at the injection coupler with a phase of π. Thus, the constraintsignal destructively interferes with input constraint signals at theinjection coupler.

However, if the constraint signal receives an overall phase shift of itafter passing through the optical gates 104 and the bus waveguide 102,the final phase shifter may apply a phase shift of π. As a result, theconstraint signal returns to an injection coupler with a phase of 0 andinterferes constructively with the input constraint signal to increasethe power in the constraint signal indicating a bit of the message iscorrupted.

At block 410, the method 400 receives, at the optical gate, a subsequentconstraint signal that includes the constraint signal that has theoverall phase shift combined with additional constraint signals, whereinthe power level of the subsequent constraint signal exceeds a powerlevel threshold of the optical gate. For example, when the constraintsignal arrives at the injection coupler with a phase shift of 0 (e.g.,an overall phase shift of π plus a final phase shift of π that isapplied to all constraint signals), the constraint signal may interfereconstructively at an injection coupler with input constraint signalsthat are being injected into the bus waveguide. The phase shiftedconstraint signal when combined with the other constraint signals maycause the overall or total power of the system to increase.

At block 412, the method 400 switches an internal state of the opticalgate in response to the power level threshold being exceeded. Forexample, if the value of the optical gate that is causing the bit erroris 1, the value may be switched to 0 to reduce the total power levelback below the power level threshold.

In one example, the value may be switched deterministically or may beswitched randomly. In one example, the blocks 404-412 may be repeateduntil the total power level reaches a steady state.

In one implementation, the method 400 may be performed by each one of aplurality of optical gates connected within a bus waveguide. The valuesstored in each one of the optical gates when the total power levelreaches the steady state may be the value associated with each bit ofthe message. At block 414, the method 400 ends.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

The invention claimed is:
 1. An apparatus comprising: a bus waveguide,wherein the bus waveguide receives a plurality of constraint signals atan injection point; a plurality of optical gates coupled to the buswaveguide, wherein each optical gate of the plurality of optical gatesoutputs an internal state via a local phase shift when at least one ofthe plurality of constraint signals has a wavelength that matches arespective resonant wavelength of the each optical gate of the pluralityof optical gates; and an injection coupler to combine the at least oneof the plurality of constraint signals with additional constraintsignals that are injected into the bus waveguide, wherein an error isdetected in a bit of a message when an overall phase shift has occurredto the at least one of the plurality of constraint signals travelingthrough the plurality of optical gates and the bus waveguide, causing apower level to exceed a power level threshold of an optical gate of theplurality of optical gates when the at least one of the plurality ofconstraint signals constructively interferes with the additionalconstraint signals that are injected.
 2. The apparatus of claim 1,wherein the power level exceeding the power level threshold of theoptical gate of the plurality of optical gates identifies the opticalgate as causing the overall phase shift and switches the internal stateof the optical gate in response to the power level threshold beingexceeded.
 3. The apparatus of claim 1, wherein a plurality of differentsubsets of optical gates of the plurality of optical gates is selectedfor each constraint of a respective check sum.
 4. The apparatus of claim1, wherein the internal state is stored in a resonator in each one ofthe plurality of optical gates and the internal state is associated witha bit value of the message associated with a respective one of theplurality of constraint signals.
 5. The apparatus of claim 1, whereinthe plurality of constraint signals comprise different wavelengths. 6.The apparatus of claim 1, wherein the optical gate that caused theoverall phase shift is identified as a location of a bit error.
 7. Theapparatus of claim 1, wherein correct message bits of the message arerepresented by the internal state of each one of the plurality ofoptical gates when a power level of the apparatus reaches a steadystate.
 8. A method comprising: receiving a constraint signal at aninjection point into a series of a plurality of optical gates;determining that the constraint signal incurred an overall phase shiftthrough the series of the plurality of optical gates, wherein theoverall phase shift is indicative of a corrupted bit of a message;increasing a power level in response to the determining that theconstraint signal incurred the overall phase shift via constructiveinterference with an additional signal; repeating the determining andthe increasing until the power level exceeds a power level threshold ofat least one of the plurality of optical gates indicating that the atleast one of the plurality of optical gates is a source of the corruptedbit; and switching an internal state of the at least one of theplurality of optical gates in response to the power level thresholdbeing exceeded.
 9. The method of claim 8, comprising: repeating thedetermining, the increasing, the repeating and the switching until thepower level reaches a steady state.
 10. The method of claim 9, wherein arespective internal state of each one of the plurality of optical gatesat the steady state is a respective bit value of the message.
 11. Themethod of claim 8, wherein the receiving at the injection pointcomprises receiving a plurality of constraint signals of differentwavelengths.
 12. The method of claim 8, comprising: applying a localphase shift by a respective one of the plurality of optical gates when awavelength of the constraint signal matches a resonant wavelength of therespective one of the plurality of optical gates.
 13. The method ofclaim 8, comprising: applying a final phase shift to the constraintsignal such that the constraint signal has a phase of 0, causing theconstraint signal to constructively interfere with additional constraintsignals that are injected, thereby causing the power level to increase.14. A method comprising: receiving, at an optical gate, a constraintsignal; determining that a wavelength of the constraint signal matches aresonant wavelength of a resonator of the optical gate; applying a phaseshift stored in the resonator to the constraint signal that contributesto an overall phase shift of the constraint signal; receiving, at theoptical gate, a subsequent constraint signal that includes theconstraint signal that constructively interferes with additionalconstraint signals, wherein a power level of the subsequent constraintsignal exceeds a power level threshold of the optical gate indicating anerror in the constraint signal; and switching an internal state of theoptical gate in response to the power level threshold being exceeded.15. The method of claim 14, wherein the resonant wavelength of theresonator is selected based on participation of a bit value stored inthe optical gate with an associated constraint of a respective checksum.